Field of the Invention
The present invention relates to the field of motor vehicles and concerns, more particularly, a method for determining the position of a portable user device in a predetermined area around a vehicle; it also concerns an associated location device and an associated portable device.
Description of the Related Art
At the present time, some motor vehicles are capable of communicating with devices carried by the users of these vehicles, for example an electronic contact key or a smartphone (or “intelligent” telephone in French).
The communication between a vehicle V (see FIG. 1) and a portable user device SD of this type makes it possible, for example, to detect the presence of the user U in a predetermined detection area ZD around the vehicle V, in order to activate certain functions of the device when the user U moves toward or away from the vehicle. By way of example, these functions may be the locking or unlocking of the passenger compartment of the vehicle V, also called “hands-free access” to the vehicle, or adjustments of equipment such as the seats, the rear-view mirrors, the air conditioning, or the like.
There may be more than one predetermined detection area, namely a first predetermined area ZD1 and a second predetermined area ZD2, larger than the first said area, defined on the basis of the distance D1, D2 between the portable device SD and the vehicle V.
The actions performed by the vehicle V differ according to whether the portable device SD is located in the first predetermined area ZD1 or in the second predetermined area ZD2.
In order to detect the presence of a portable user device SD in the detection area ZD, the vehicle V periodically transmits, via an antenna A, a radio signal including what is known as a broadcast message. If the portable user device SD receives this signal via its integrated antenna, it measures the power of the signal, which in a known way is referred to as the RSSI (“Received Signal Strength Indication”) and sends this value to the vehicle V in a transmission signal. The vehicle then uses this power value to estimate the distance D between the location of the portable user device SD and the vehicle V, and thus to determine whether or not the user U is present in the detection area ZD.
However, it has been found that the human body may have a negative effect on the performance of the antenna of the portable user device SD, depending on its position relative to the human body. This is because the antenna of the portable user device SD may be oriented in a random and arbitrary way during its approach to the vehicle V, and the tissues of the human body may absorb some of the radio signals transmitted or received by the antenna of the portable user device SD, causing an antenna impedance mismatch, resulting in a loss of power radiated toward the body of the user U which may, for example, be as much as 25 dB.
The power loss caused by the position of the portable user device SD on the body reduces the value of the RSSI, measured by the portable user device SD and sent to the vehicle V, so that the estimated distance values D may be incorrect, resulting in errors in the detection of the presence of the user U in the detection area ZD, causing a considerable problem.
A disparity has also been found in the RSSI measurement performance depending on the type of smartphone used.
There is an increasing variety of smartphones on the market, meaning that there is also a variety of electronic circuits for measuring the RSSI value.
More precisely, the gain of the receiving antenna of the smartphone and the amplification of said measurement differ from one smartphone to another. This gives rise to a non-negligible error in the RSSI measurement performed by the smartphone, and therefore in the estimated distance D between the smartphone SD and the vehicle V. Since the value of the distance D is erroneous, the actions performed by the vehicle V are no longer appropriate to the true distance D between the portable device SD and the vehicle V. Thus the vehicle V may issue an unlocking command when the portable device SD is actually located too far from the vehicle V for this action.
This is illustrated in FIG. 2. FIG. 2 shows the estimated values of distance D according to the RSSI values measured by three different types of smartphones, SD1, SD2, and SD3.
It is evident from this graph that a measured RSSI value “5”, for example S==68 dB, corresponds to three different estimated distances DS1, DS2, DS3, between the smartphone SD and the vehicle V, depending on the type of smartphone SD1, SD2, or SD3 that made the RSSI measurement.
For the first type of smartphone SD1, the estimated distance DS1 is 4 m; for the second type of smartphone SD2, the estimated distance DS2 is 6 m; and for the third type of smartphone SD3, the estimated DS3 is 25 m.
Vice versa, for a given distance threshold Dx between the smartphone SD and the vehicle, the variation ΔdB of the corresponding RSSI measurement may reach 30 dB.
This variability in the RSSI measurement for the same distance between the portable device and the vehicle also appears, although to a lesser degree, when the portable user device is a hands-free access badge.